Consider a random walk on a one dimensional space with states {A, B,C, D}, which in general moves left and right with respective probabilities q and p. If the walker is at state A, it will transit to state B on the next step. Analogously, if the walker is at state D, it will transit to state C on the next step. Formulate the transition probability matrix of this random walk by taking into account the boundaries that may exist. Are there any absorbing states in this Markov chain?